【作者】 徐令宇；

【导师】 王国新； 鵜飼惠三；

【作者基本信息】 大连理工大学 ， 防灾减灾工程及防护工程， 2013， 博士

【摘要】 桩基础在当前日益发展的土木工程基础设施建设中得到了广泛的应用,而桩基础不仅仅承担着竖向荷载,同时也会承担水平向荷载,特别在地震过程中,桩头所支撑的上部结构会产生很大的惯性力,使得钢筋混凝土桩很容易进入非线性状态,从而产生破坏,因此,如何合理地建立水平加载条件下考虑钢筋混凝土桩的材料非线性的静力分析方法是桩基础抗震设计中重要的研究课题,对于软土地区的水平加载桩,桩与其后侧粘土的分离对桩基础的水平承载性能有很大影响,如何在计算中考虑这一影响尚待研究,同时,对于软土地区的桩基础会涉及到地基改良问题,如何确定经济有效的改良深度也是个研究课题,另外,目前对于群桩基础的水平承载力设计方法还不够完善,需要做进一步研究,同时,在地震过程中,饱和砂土场地会产生砂土液化现象,而目前对于液化场地上群桩基础抗液化性能的研究甚少,也是需要进一步开展的工作。本文围绕着桩基础做了以下研究工作：(1)针对现有的应变楔形体模型存在的缺点,如楔形体前破坏面不连续、应力应变路径的公式比较复杂且参数不易选取、不适用于超固结粘土的计算等,本文提出了修正的应变楔形体模型以用于桩基础的水平响应的计算,编制了SWPILE有限元程序,并通过已有文献中大量的砂土、粘土以及层状土中水平加载桩试验,验证了该修正模型的可行性,并讨论了应变楔形体模型中的收敛问题、楔形体中应变和楔形体高度变化规律以及参数的敏感性等,最后,利用该修正的应变楔形体模型研究了p-y曲线的影响因素；(2)基于修正的应变楔形体模型,研究了双层地基(密砂土和软粘土)中桩基的水平响应,并研究了土的成层效应对桩基水平响应的影响规律,包括密砂土中的软粘土夹层和软粘土中密砂土夹层,在此基础上,分析了密砂土场地和软粘土场地中地表土层最大影响深度及其与计算出的楔形体高度的关系；(3)在修正的应变楔形体模型中,采用纤维单元实现了水平加载条件下考虑钢筋混凝土桩的材料非线性的计算,并根据已有文献中的水平加载桩试验资料和计算结果,验证了该计算方法的可行性。利用该方法研究了钢筋混凝土桩截面刚度在水平荷载作用下的变化规律以及桩的材料非线性对桩的挠度以及最大弯矩的影响；采用了单元应力磨平的方法把纤维单元高斯点的应力恢复到截面网格节点上,并绘制了截面的应力云图,从而了解了中性轴在不同荷载下的变化以及混凝土开裂的发展情况；(4)在修正的应变楔形体模型中引入了p-y乘子的概念来考虑群桩效应,并综合了应变楔形体模型和Mokwa等效单桩法的优点,提出了改进的等效单桩法,通过已有文献中的水平加载单桩与群桩实验,验证了该方法的有效性,并基于该方法,研究了p-y乘子对不同位置(排)桩工况中计算出的p-y曲线的影响,并分析了不同深度处以及不同位置(排)桩工况中计算出的粘土和砂土的极限地基反力,还研究了水平荷载和场地深度对群桩效应的影响以及不同桩头条件对单桩和群桩基础水平响应的影响；(5)初步研究了液化场地桩基础水平响应计算方法。基于完整的震后房屋损坏调查资料,应用完全耦合的动力有限元方法再现了由砂土液化引起的房屋破坏情况,利用修正的Pastor-Zienkiewicz Mark-Ⅲ模型来模拟砂土在地震荷载作用下的液化特性,用三轴试验结果和标准贯入数据来确定该模型参数,根据一组竖向分布的加速度传感器记录,采用SHAKE91程序确定了地震动输入,并把有限元计算出的地表水平位移与已有文献中经验公式计算出的结果以及震害调查的结果进行了比较,从而验证了有限元计算结果的合理性,随后,通过一系列的工况分析了边坡对房屋震害的影响,并对比研究了群桩、水泥土、排水系统的抗液化性能,为液化场地上群桩基础设计提供了计算方法和参考依据。更多还原

【Abstract】 Pile foundations have been widely used in engineering with the growing construction of civil infrastructures. Piles are commonly used to transfer vertical (axial) forces, arising primarily from gravity (e.g., the weight of superstructures). However, it is not only the axial force that the piles carry; often the piles are subjected to lateral (horizontal) forces and moments. Especially during the earthquake, large lateral forces and overturning moments are imposed on the piled foundations by the inertia of superstructures. As a result, the plasticity can easily occur for reinforced concrete piles as the tensile strength of concrete is quite low. This can lead to bearing failure and cause heavy casualties and severe economic losses. Thus, it is of great demand in seismic design of pile foundation that quasi-static computation method for response of laterally loaded reinforced concrete piles should be established. In addition, the effect of gap formation between the pile and the clay on the response of laterally loaded piles and the mechanism of the gap formation need to be studied and the computation method for the design of laterally loaded pile group also needs to be improved. Finally, liquefaction is a common phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking and piles are usually used to mitigate the adverse effect of liquefaction, however, little research has be conducted on the liquefaction-mitigation performance of piled foundations. Thus in this thesis, some works related to these issues are conducted and shown as follows:(1) Due to the limitations in the current strain wedge model, a modified strain wedge model is proposed to analyze laterally loaded piles by using finite element software SWPILE compiled in Fortran. A number of full scale pile tests in sand, clay, and layered soils are used to verify the applicability of the proposed method. In addition, some discussions are made on convergence of the proposed modified strain wedge model, the strain wedge depth and the soil strain in the strain wedge for a variation in the lateral load, the sensitivity of some input parameters, the hyperbolic and bilinear stress-strain relationships, and so on. Finally, the influential factors of p-y curves are studied based on the proposed method.(2) On the basis of the modified strain wedge model, layering effects of soils on the response of laterally loaded piles are investigated by comparing pile behaviors in uniform sand and clay soils with those both in double-layer soils and in triple-layer soils (i.e.. sand layer in clay deposit and clay layer in sand deposit). Then, the most influential height of both sand soils and clay soils at the ground surface is studied based on a number of case studies. Finally, the relation between the most influential height and the stain wedge height is investigated.(3) Computation method for analyzing nonlinear behavior of laterally loaded concrete piles is proposed using fiber element. The versatility of the proposed method is demonstrated by a number of full scale pile tests. The effects of material nonlinearity of pile on flexural rigidity, load-deflection curves, and load-maximum moment curves are adequately studied. In addition, the stresses on gauss points are recovered to nodes of the grid of pile section and then depicted in order to study the neutral axis of the pile cross-section and the development of plastic-state area on the pile cross-section with the variation in the lateral load.(4) Simplified computation method for the response of laterally loaded pile group is proposed by the combination of the concept of p-y multiplier proposed by Brown et al.(1988) and group-equivalent pile procedure proposed by Mokwa (1999). The proposed method is verified by a number of full scale pile-group tests. In addition, the effect of p-y multiplier on the calculated p-y curves, the effect of the boundary conditions of pile head on the lateral response of piled foundations and the effect of lateral load and soil depth on the group effect are investigated.(5) A preliminary research is conducted on lateral response of piled foundations at liquefied site. In addition, a fully coupled dynamic effective-stress finite element procedure UWLC is used to reproduce the damage of the two houses due to dune liquefaction. A modified Pastor-Zienkiewicz III model was used to describe the liquefaction behaviors of the younger sand dune layer. The parameters of the model were quantified by the laboratory tests on undisturbed samples and the Standard Penetration Test data. The input motions for the numerical analyses were calculated by SHAKE91using the seismic motions recorded by the vertical array at the service hall of Kashiwazaki-Kariwa Nuclear Power Plant. First, the lateral displacements of the ground surface calculated by UWLC were compared with those calculated by the method of Zhang et al.(2004) to verify the reliability of the UWLC results. Second, the effects of the sand dune slope on the damage to two houses were investigated. Finally, the effectiveness of each countermeasure used for house B and the combinations of two countermeasures was studied. The research highlights the liquefaction-mitigation performance of pile-group foundation.更多还原